The present invention relates to a magnetic disk drive and more particularly to a magnetic head positioning technique in a high track density.
In conventional magnetic disk drives, a magnetic head to write and read information is disposed above a magnetic disk medium used to store information and is supported on an actuator and moved in a radial direction from one track to another on the magnetic disk medium. Position information to position the magnetic head at a target track is recorded as servo information on the magnetic disk medium. The servo information is normally recorded at locations different from those of data or information stored by the user. The magnetic head detects this servo information at predetermined intervals. The magnetic disk drive locates a target track based on the detected servo information and performs control to place the magnetic head at a particular position. That is, a position signal detected by the magnetic head must include a position signal that is obtained by reading the servo information. Only when the magnetic head flies over the servo information area, can the position of the magnetic head be determined. The magnetic head cannot obtain information about its position when the head is moving over a data area. For this reason, if the magnetic head 101 should suddenly deviate from the correct position while reading data from a data area, for example, the position deviation cannot be detected making it impossible to prevent a data error.
As the servo information increases, the time interval at which to detect the position signal becomes short and the positioning can be made with higher accuracy. When the track density is to be increased, the positioning control requires higher precision and it is common practice to increase the amount of servo information.
A track offset can be detected by comparing amplitude values of a plurality of burst signals recorded in the servo information and has conventionally been detected to perform the positioning control. The track offset refers to a distance by which the center of the magnetic head is deviated from the center of a target track during reading or writing. Hence, when a track offset occurs when the magnetic head is not in the servo information area but in the data area where a signal corresponding to the data is stored, the amplitude of the read signal decreases, causing such troubles as an increased bit error rate. The larger the track offset, the greater the changes of these values are.
The conventional technique has the following problems.
1) For a more precise positioning control, the servo information needs to be increased, which in turn increases the servo information areas 302 (FIG. 3). However, because the servo information and data are recorded on the same plane, an increase in the servo information results in a decrease in the data area 303 and therefore the storage capacity of information written in the magnetic disk medium.
A technique for generating an index sector pulse signal in the data area is disclosed in Japanese Patent No. 3042790.
2) Another problem is that in the method of detecting the position signal representing the position of the magnetic head from only the servo information, the position signal cannot be detected when the data is read or written. Hence, in a system where the actuator is controlled based on the servo information, when vibrations or impacts having frequencies higher than a sampling frequency of the servo information is applied as disturbance to this servo control system, the positioning control cannot be performed in principle, leading to data errors. To solve this problem requires increasing the sampling frequency of the servo information. As described above, however, this reduces the data area 303 and therefore the data storage capacity of the magnetic disk drive.
In other words, the problem that needs to be addressed is that the only time the position of the magnetic head can be detected is when the magnetic head flies over the servo information area 302 from which the servo information can be read and that the magnetic head 101 (FIG. 10) cannot obtain information on its position when it is over the data area 303. Thus, when the magnetic head 101 is over the data area, as when it is reading data, if the magnetic head 101 suddenly deviates from its correct position, this deviation cannot be detected.
3) Further, the track offset on the magnetic disk medium can occur in either direction toward an outer circumferential side or an inner circumferential side. So, if the occurrence of the track offset can be detected and its absolute value estimated, the direction of track offset cannot be determined, making it difficult to use the track offset as an object to be monitored. For example, FIG. 11 shows how the read signal of the magnetic head 101 of the magnetic disk drive changes with respect to the track offset. That is, when the magnetic head is positioned at the track center, the output value of the information read signal is maximum. When the magnetic head is deviated from the center, the output value decreases. When it is completely off track, the output value is shown to be zero.
The magnetic disk drive has a function to oscillate the magnetic head when it is moving over the data area and a function to detect a read signal when oscillating the magnetic head and, from the detected read signal, locate the position of the magnetic head. The magnetic head is then controlled to be moved to a target track.
The oscillation refers to the magnetic head making periodic motion relative to the head suspension, slider or magnetic disk medium. The direction of oscillation of the magnetic head need only be roughly in a radial direction of the magnetic disk medium and some deviations are permitted.